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IT WORKS!

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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IT WORKS!

Unread postAuthor: ralphd » Thu Jun 25, 2009 9:58 am

I finally got my first pneumatic piston working. Many thanks to all who suggested the bigger port; that was it. It's much quieter than my combustion as well. That's a huge plus here in the city. No more calls to the police about someone firing a shotgun. It also launches tennisballs almost as far. I only charged to 70psi.

I won't bore you with the complete details but in a nutshell...2 inch resovoir ~3 foot long hooked up to a piston housed in a 2 inch "T" with a simple 1/2 inch ballvalve(previously a 1/4inch blowgun) married to a 2 1/2 inch barrel 2 foot long.

I guess it's the sudden/quick loss of pilot pressure which moves the piston or at least mine. The blowguns must work fine with a diaphragm used on both the sprinkler and Supah valves.

A few question came up though. Can anybody help?

1.) Is the bumper behind the piston necessary?
2.) Must the Piston be lubed?
3.) If so, with what? Vasoline? It seems I'm never without it.
4.) What pressures are you using out there? Safely

Once again thankyou. Now that I got the prototype done I will most definitely mod it for a while then sell it to fund my next project. More likely a launcher with a diaphragm.
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Last edited by ralphd on Thu Jun 25, 2009 11:49 am, edited 2 times in total.

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Unread postAuthor: LikimysCrotchus5 » Thu Jun 25, 2009 10:06 am

Well good for you!

1. Well it doesnt have to be attached to your piston but it is necessary because that absorbs the shock when it shoots back and if thats not there to absorb that shock then the fitting will absorb the shock which can damage or break the gun.

2. Lubing the piston makes it easier for the piston to actuate and it will increase performance because it will actuate quicker as well.

3. Well with pvc i have used 100-120 but no more than that. If all the parts are pressure rated, then you should assume that the fittings havee the same psi rating as the chamber pipe. As long as you dont go up to the rating of the pipe, then im sure it will be safe.
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Unread postAuthor: sniper hero » Thu Jun 25, 2009 10:10 am

bumper: depends on the material of the piston
if it is e.g. hot glue there is no bumper needed but for a piston made from wood I would recommend it
lubricant: very usefull because it makes the piston move more smoothly and quicker
it also helps to seal
pressure: I used a hot glue piston on 40 bar
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Unread postAuthor: psycix » Fri Jun 26, 2009 2:19 pm

Bumper:
A rubber piston in a metal T piece will not need a bumper.
A pvc piston in a PVC piece for example does. I broke my PVC piston on the first shot (30 psi) thinking "ah it wont need a bumper on that pressure yet"
On high powered cannons, also the PVC T-piece may take a good hit when the piston doesnt have a bumper. You dont want this T to crack open and your piston to come out.
Bumpers may be either attached to the piston, downside->heavier piston.
Or to the back of the T-piece, downside-> There has to be a free way for the air to the pilot valve

Lube:
Lubing is good. When working with PVC avoid things that could soften the plastic. Also make sure your lube isnt flammable. WD-40 could go combusting.
For lube, I would recommend KY jelly.
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Unread postAuthor: Hubb » Fri Jun 26, 2009 2:49 pm

3.) If so, with what? Vasoline? It seems I'm never without it.

Am I the only one who caught that?

As far as your questions, go, I guess I'll give my two cents worth:
1) The bumper may not be necessary, but it won't hurt it either. It's best to have it and not need it than to need it and not have it. Understand? I've used o-rings and pieces of rubber hose to good success.
2) The piston doesn't necessarily need lubrication but, again, it won't hurt it.
3) Vasoline is oil based. It is probably not the best lube for PVC. KY jelly is water based and is a much better choice. Also, consider graphite powder and even Rain-X, just make sure that it is safe for the rubber and other materials used in the valve.
4) I've used up to 120psi in my 2" piston. But, I will only suggest using pressure that you are comfortable with.
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Unread postAuthor: ralphd » Fri Jun 26, 2009 2:51 pm

I added a bumper. I used two rubber disks on the back of the piston. These are the same disk which I used on the front as the seal. The first material I tried was some foam rubber but it collapsed when I opened the valve and plugged the hole. The piston is made of plywood and weighs 4.8 ounces. I lubed it as well with silicone grease. I didn't see any greater performance but I am a little cautious, only 80psi. It is a little louder though. Later on I will probably try a larger valve or maybe a QEV.
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Unread postAuthor: Technician1002 » Fri Jun 26, 2009 3:22 pm

psycix wrote:Bumpers may be either attached to the piston, downside->heavier piston.
Or to the back of the T-piece, downside-> There has to be a free way for the air to the pilot valve


I have not had any trouble with this bumper style. The valve is not a traditional "Piston" but is the QDV piston. I have a series of photos of items as they load in from the muzzle end. This bumper has never blown out the Pilot hole. My large and small cannons use the same layout, QDV and traditional Piston.

This foam bumper is cut from a garden knee pad. It is closed cell foam and pretty tough stuff.

The QDV has to be hand pulled open, which is why the bolt runs through it. The hole in a bumper and stop for the pilot would replace the bolt in a regular piston. A flat backed piston distributes the force evenly.

The Mouse Musket uses a traditional piston. The flat back and ring bumper are still used.
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Attachments
Stopper.jpg
Stopper limits the distance the piston can fly open. This limits the speed at which it hits the bumper.
FoamBumper.jpg
Foam bumper with moderate size pilot hole. Acts as a "Dashpot" when the piston compresses the air in this space.
Piston.jpg
Flat bottom piston is next. It is free to slide on the bolt.
Assembled.jpg
A nut is added to hit the piston open to where air takes over and blows it open.
PICT250.JPG
Mouse Musket piston and bumper.
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